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News Navigator: What has caused Japan's heat waves this summer?

People walk under an umbrella to block the sunlight in Chuo Ward in the city of Hamamatsu, Shizuoka Prefecture, on Aug. 17, 2020, when the local temperature hit 41.1C. (Mainichi/Yuki Miyatake)

The Mainichi Shimbun answers some common questions readers may have about the mechanism behind this year's hotter-than-average summer in Japan.

    Question: We have had another hot summer in Japan, haven't we?

    Answer: Yes. Temperatures rose nationwide from the beginning of August, and the mercury has hit 35 degrees Celsius or higher for days at a time in a wide area from the Kanto region in eastern Japan to the southwestern island of Kyushu since mid-August.

    Q: How did it get so hot?

    A: According to the Japan Meteorological Agency, a Pacific High pressure system centered over the ocean southeast of Japan spread to cover wide areas of the country. Furthermore, a Eurasia continental Tibetan high-pressure system slid in over the Pacific High, creating a double-decker high-pressure system over Japan.

    Under a high-pressure system, skies remain clear and clouds are unlikely to form, making it easier for the heat to accumulate on the ground due to the increased intensity of solar radiation. The overlapped high-pressure systems boosted that effect, causing temperatures to soar.

    Q: How did they overlap?

    A: The subtropical westerlies running north of the Tibetan system meandered further north and closer to Japan than in a usual year, allowing the system to extend over the country. It also developed further due to increasing water temperatures in the Indian Ocean, covering the Japanese archipelago more extensively than normal.

    Q: Did all those circumstances cause 40C or hotter temperatures in some places?

    A: There were reportedly some contributing factors unique to the hottest areas. In Chuo Ward of central Japanese city of Hamamatsu and surrounding areas, where the high reached 41.1C on Aug. 17, tying the country's hottest temperature on record, the air over the area was warmer than the surrounding skies, with the northwesterly wind blowing, which prevented cool oceanic air from reaching the city. The air warmed up in the Nobi Plain, which stretches northwestward, apparently traveled and created a strong downslope wind when it passed mountains to the west of Hamamatsu, fueling the temperature spike.

    Q: Has it been warmer than usual in other places of the world?

    A: Temperature in Verkhoyansk, northern Russia, hit 38C on June 20, breaking the record in the Arctic, and the mercury in Death Valley in California reached 54.4C on Aug. 16. The number of days we experience extreme heat has been increasing on a global scale, and scientists point to global warming as a cause.

    (Japanese original by Tomoko Mimata, Science and Environment News Department)

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